1. Technical Field of the Invention
This invention relates generally to wireless communication devices and more particularly to radio interfaces of such wireless communication devices.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies then. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
In many wireless applications, a radio transceiver includes one antenna that is shared by the receiver section and the transmitter section. The sharing of the antenna may be achieved by a transmit/receive switch or a transformer balun. In recent advances in the wireless communication art, a transformer balun has been implemented on-chip with the receiver section and the transmitter section. In such an implementation, a single-ended winding of the transformer balun is operably coupled to the antenna, a first differential winding of the transformer balun is operably coupled to the receiver section, and another differential winding of the transformer balun is operably coupled to the transmitter section.
While the benefits of using an on-chip balun are many, there are some issues of concern, which include impedance matching of the loads on the single-ended winding and on the differential winding, efficient energy transfer from the transmitter section to the antenna via the transformer balun, and complexity of implementation. Such issues arise, at least in part, due to the loading of the transformer balun varies depending on whether the transceiver is in a transmit mode or in a receive mode. For example, the output impedance of the power amplifier, which is a load on the differential winding, varies depending on whether the power amplifier is active or inactive. Further, the input impedance of the low noise amplifier, which is a load on the differential winding, varies by a factor of two or more depending on whether the low noise amplifier is active or inactive.
FIG. 1 is a schematic block diagram of a known low noise amplifier (LNA), which exhibits a variable input impedance. As shown, the LNA includes a differential architecture wherein a differential RF signal is received at the LNA inputs N and P and produces a differential output at LNA OUT N and P. When the LNA is active, the input impedance of the LNA is based on the transconductance (gm) of the input transistors, the parasitic capacitance of the input transistors, and the series inductance between the source of the input transistors and ground, which may be a separate inductor or parasitic inductance of the coupling. When the LNA is inactive, its input inductance is based on the parasitic capacitance and the series inductance. As such, the input impedance of the LNA varies significantly depending on whether the LNA is active or inactive.
Therefore, a need exists for a low noise amplifier that has a substantially constant input impedance regardless of whether it is active or inactive such a radio front end may provide efficient energy transfer from the transmitter section to the antenna, provide enhanced impedance matching, and reduce the complexity of implementation.